In metal casting industries, including aluminum casting, low pressure die casting is frequently performed, including a technique known as "Low Pressure Permanent Mold" (LPPM) processing. As shown in FIG. 1, this process uses what are called riser tubes 10 as a conduit for the molten metal 4 to pass from the melt chamber 1 to the mold cavity 2a of mold 2. The mold 2 sits atop the melt chamber and is fastened to the top of the riser tube 10 which extends downward into the bath of molten metal 4. Because the molten metal 4 is forced to rise up the core of the tube, these tubes are also referred to as "stalks". A sectional drawing of a typical riser tube is shown in FIG. 2. The casting operation is achieved by applying a positive gas pressure, usually 10 to 30 psi., to the surface of the bath of molten metal. The pressure forces metal into and up the length of the riser tube and into the mold cavity. The tube must be nearly gas-tight for two reasons:
1) to prohibit gas from becoming entrained in the molten metal (caused by the Venturi effect where gas is siphoned into the tube through the tube wall by the motion of the molten metal), resulting in gas voids in the finished metal casting, and PA1 2) to maintain a positive pressure differential between the outside of the tube (inside the melt chamber) and the inside of the tube. PA1 1) near impermeability to air at application temperature so that the applied pressure acts on the molten metal and does not take the "path of least resistance" through the tube; PA1 2) non-reactivity with the molten metal being cast, to yield high purity metal castings and to enhance life of the riser tube; PA1 3) controlled thermal conduction and insulation so that as the metal is cast into the mold and allowed to solidify, the tube allows the metal to remain in a molten state which allows back-flow and drainage of the tube; and PA1 4) controlled mechanical properties so that as pressure is applied to the tube/mold cavity interface to ensure a tight enough seal (to prevent molten metal leakage), the tube is not damaged and can therefore be used again.
It is this pressure differential that causes the molten metal to rise up the tube into the mold cavity. Upon filling the cavity, the molten metal is allowed to solidify in the mold and form the casting. The pressure is subsequently released and the molten metal remaining in the stalk is allowed to back-flow out of the tube, draining back into the melt.
Thus, the desired properties of a riser tube used in this application include the following:
Traditional riser tubes are currently formed by machining metal blanks into the desired geometry, or forming ceramic tubes (e.g.: silicon nitride, SiAlON, aluminum titanate, fused silica) using modern conventional processing techniques. The metal riser tubes that have been functionally utilized for many years ensure gas impermeability. These metal tubes can be made of a variety of materials including basics like iron or steel, or exotics such as titanium alloys. Iron or steel riser tubes tend to contaminate molten metals such as aluminum via alloying, and likewise may yield lower quality metal castings. In fact, as an additional maintenance step, many die casting end-users coat these iron/steel tubes after each change-out in an attempt to curtail finished metal casting contamination. Titanium or other exotic metal alloy tubes may not be reactive with molten aluminum, but are quite expensive.
Fused silica ceramic riser tubes are frequently used, but lack mechanical strength to survive typical handling techniques in a casting facility. In the case of molten aluminum, silica is reactive with this metal, and hence, the molten metal may pick-up contamination and the life of the tube is shortened. Also, these fused silica tubes are typically gas permeable thus providing sub-optimal stalk performance and metal casting quality. The currently used more exotic ceramic riser tubes such as silicon nitride, SiAlON, and aluminum titanate are generally inherently nearly gas impermeable, but are also expensive due to high raw material and processing costs.
Typically, low pressure die casting systems are utilized a high proportion of available time, and thus require regular maintenance and monitoring. Commonly, at some time interval (or number of cycles interval), riser tubes are removed from the die casting apparatus, allowed to cool, cleaned (molten metal peeled off), and then are reinstalled. In some cases, including with fused silica ceramic and iron/steel metal, they are also sometimes coated in some manner before reinstallation. Frequent handling of this nature necessitates a riser tube material with a reasonable degree of mechanical strength. Exotic alloys and composites provide adequate thermal properties but lack the mechanical strength to survive simple mishandling such as an accidental minor hit against a building wall, etc.
Thus, consideration was given to utilize a riser tube material of appropriate thermal conductivity to maintain the metal in a molten state, and also to have reasonable mechanical properties to survive the necessary rigors of normal industrial use. There are some ceramic materials that can fulfill those requirements. However, there are no known economical ceramic materials that are also nearly gas impermeable, which is a key characteristic in this application, as has been explained.